Various fossil fuels contain components other than hydrogen and carbon, such as sulfur. Combusting such fuels may emit objectionable amounts of gaseous sulfur compounds into the atmosphere. For example, petroleum, coke, coal and heavy residue from hydrocarbon refining processes often contain relatively high contents of sulfur and nitrogen compounds. In addition, some liquid and gaseous fuels contain relatively large amounts of sulfur compounds. Such liquid fuels, while unsuitable for use directly in furnaces, may be processed to remove objectionable components. Solid fuels are generally more difficult to process to remove undesirable sulfur and nitrogen compounds and are less convenient to transport than liquid and gaseous fuels.
It is well known that various carbonaceous fuels (i.e., fossil fuels) may be converted to reducing gas, fuel gas, or synthesis gas (syngas) comprising carbon monoxide and hydrogen by partial oxidation at an elevated reaction temperature and pressure. In these processes, a fossil fuel, such as coal, is reacted with an oxygen-containing gas, usually commercially pure oxygen, in a closed, compact reactor at an autogenous temperature within a range of about 1000.degree. to 1600.degree. C. The reaction zone is usually maintained at a pressure above about 100 pounds per square inch gauge (psig) and may be as high as 3000 psig; usually the process reaction pressures during steady state operation are in the range of 200 to 1200 psig. Steam may be introduced into the reaction zone to assist in the dispersion of fuel in the reactor. Steam also assists in control of the reaction temperature and acts as a reactant thereby decreasing the amount of free oxygen required in the process.
It is also well known that synthesis gas mixtures comprising carbon monoxide and hydrogen are important and useful commercially for a number of reasons. For example, synthesis gas has been used for many years as a source of carbon monoxide in carbonylation reactions. A method for producing such mixtures of gases, which has been used commercially for decades, is by the partial oxidation of sulfur-bearing hydrocarbon fuel; generally, coal in the form of a slurry, which yields a product gas comprising CO, CO.sub.2, H.sub.2 and H.sub.2 S.
Fuel burners for producing synthesis gas by partial oxidation of a coal slurry are known. See, for example, U.S. Pat. Nos. 3,758,037 and 3,945,942, which relate to a multitube burner assembly and a process for its use.
The gases produced from gasifying a fossil fuel such as coal may be, and preferably are, further processed to separate them and remove by-products. This is accomplished by gas purification and processing systems known in the art. For example, the product gas from a gasifier may be processed to remove CO.sub.2 and H.sub.2 S to levels of 1-2 ppm. The removed H.sub.2 S may be further processed to convert it to its elemental form in a sulfur recovery plant; the CO.sub.2 may also be captured and/or further processed. After removing CO.sub.2 and H.sub.2 S, the remaining gas primarily contains CO and H.sub.2 (i.e., syngas). The syngas is most often sent directly to downstream plants for further processing into useful derivatives, such as acetyl chemicals like acetic acid and acetic anhydride.
Under steady state conditions in a typical partial oxidation gas generation plant, the coal gasification process makes syngas and the gas purification system minimizes emissions of contaminants. However, because of the nature of the processes (gas streams instead of liquid, wide temperature extremes, and high pressures), the processes must be started in sequence. The gasifier is started first and must be started instantly at a 50% rate because the burner within the gasifier cannot "turn down" past 50%. Since the product gas from the gasifier cannot be stored, it must be vented and burned at a flare stack until the gas purification process (i.e., the gas clean-up process) can be pressurized, cooled down, and started up.
In a conventional start-up of a partial oxidation gas generating process the gas generator is started at atmospheric pressure after preheating to at least 950.degree. C. Until the gasifier is pressurized and downstream processes brought on-line the resulting effluent, comprising syngas, is burned in a flare. As is well known to those skilled in the art, this results in higher than normal emissions of contaminants such as sulfur. See, for example, U.S. Pat. No. 4,385,906; see also U.S. Pat. No. 3,816,332.
Thus, it is well known to those in the art that start-up of a partial oxidation gas generator presents special challenges, including contaminant emissions. For example, U.S. Pat. No. 4,378,974 to Petit et al. discloses a start-up method for a coal gasification plant, in particular a refractory lined rotary kiln. The method of Petit et al. focuses on the problems that arise from coal having a high chlorine content. In Petit's reactor, the lining is made of materials susceptible to chlorine-induced cracking in the presence of oxygen. Petit starts the reactor up in stages while maintaining an oxygen content in the reactor of a sufficiently low level to prevent chlorine-induced cracking of the refractory lining.
In addition, U.S. Pat. No. 4,385,906 to Estabrook discloses a start-up method for a gasification system comprising a gas generator and a gas purification train. The method of Estabrook isolates and prepressurizes the gas purification train to 50% its normal pressure; the gas generator is then started, and its pressure increased before establishing communication between the generator and the purifier. Purified gases from the purifier may then be burned in a flare until all parts of the process reach appropriate temperature and pressure.
We have found that air contaminants, such as sulfur, which are characteristic of start-up, may be eliminated by starting the gasifier on a sulfur-free, liquid organic fuel. In our unique process, a gasifier is started using a sulfur-free liquid organic fuel; once the appropriate conditions of temperature and pressure are attained in the gasifier and gas purification systems, the burner is transitioned to a carbonaceous fossil fuel slurry.
A number of factors must be considered before determining the appropriate liquid to use in the improved process for starting a partial oxidation gas generator. Those considerations included:
1. Fuel Considerations PA1 2. How to transition from liquid fuel to slurry without interrupting the feed system. PA1 3. How to start-up sulfur recovery plant at low H.sub.2 S concentrations before the system is lined out.
a) oxygen to fuel ratio must be similar (on volume basis) as that present when the coal slurry is fed to the gas generator. PA2 b) Resulting gas composition should have similar CO/H.sub.2 /CO.sub.2 ratios so that downstream plants can operate within their designed parameters. PA2 c) The liquid fuel should be compatible with slurry since both will use common equipment during transition. PA2 d) The liquid fuel should be "clean" and free of substances that are not normally present such as high concentrations of metals. PA2 e) Atomization differences between the two fuels on the same burner. PA2 f) Gasifier dynamics such as flame temperature, refractory wear, pressure build-up. PA2 g) Fuel availability, cost, health and safety hazards.
As set forth in more detail below, the sulfur free liquid organic fuel may be a hydrocarbon compound of 1 to 20 carbons.
It is an object of the present invention to provide a process for start-up of a partial oxidation gas generator that minimizes contaminants such as sulfur.
It is a further object of the invention to provide a start-up procedure that does not require raw syngas derived from a fossil fuel to be flared and vented to the atmosphere.
It is also a further object of the invention to provide an alternative process for preheating a partial oxidation gas generator that does not require a natural gas source.